CDC is funding studies with the following objectives: a) To compare the safety and antigenicity of three doses of DTP administered at 2, 4, and 18 months of age to four doses of DTP administered at 2, 4, 6, and 18 months. b) To evaluate the safety and antigenicity of DTP administered to premature infants compared to full-term infants. The serologic evaluations will be done at CBER. Study design called for a total of 500 children to be enrolled and 2500 blood specimens to be collected. After approximately 80 children had completed the study, preliminary serological analyses were performed by Dr. DeForest using agglutination and CHO-cell neutralization assays. These revealed that the 3-dose group had antibody titers significantly lower than the 4-dose group. The study was stopped at that time. The Laboratory of Pertussis performed ELISA assays on these samples to determine if ELISA assays would also demonstrate reduced antibody levels in the 3-dose group. Approximately 400 sera were tested and 1600 test results were reported.